System and method of wavelength add/drop multiplexing having client configurability

ABSTRACT

An optical carrier drop/add transmission system and method for adding a signal to multiplexed input optical signals conveyed by an optical multiplex input line. The multiplexed input optical signals are demultiplexed to provide isolated input optical signals to an optical switch matrix comprising switches in an array of lines and columns, the isolated input optical signals being inputted in a direction parallel to a line of switches in the optical switch matrix. The added optical signal is input in a direction parallel to a column in the optical switch matrix. An output line is selected and the switch that is on the column on which the added optical signal is inputted and on the selected output line is switched.

This non-provisional application claims the benefits of U.S. ProvisionalApplication No. 60/172,732 entitled “Wavelength add/drop MultiplexerWith Client Configurability” which was filed on Dec. 20, 1999 and ishereby incorporated by reference in its entirety. The applicants of theprovisional application are Evan L. Goldstein, Lih-Yuan Lin and RobertW. Tkach.

This non-provisional application also claims the benefits of U.S.Provisional Application No. 60/204452 entitled “Micro-machined OpticalAdd/Drop Multiplexer With Client Configurability” which was filed on May16, 2000 and is hereby incorporated by reference in its entirety. Theapplicants of the provisional application are Chuan Pu, Evan L.Goldstein, Lih-Yuan Lin and Robert W. Tkach.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to optical communication. More particularly, thisinvention relates to systems and methods using optical switches foradding and dropping channels from an optical transmission medium.

2. Description of Related Art

In current optical communication systems, multiple channels aremultiplexed onto a single optical transmission medium using multiplexingtechniques, such as wavelength-division-multiplexing (WDM). WDM cancombine a plurality of communication channels, in the form of discretewavelengths, onto a single optical fiber. As multiplexing techniquesimprove, an increasing number of channels are being transmitted on asingle optical fiber or group of optical fibers. As the number ofchannels increase, so too does the need for an ability to add and/ordrop a portion of the channels to and/or from the transmission medium.

Current communication systems can use an opto-electronic regenerationtechnique to add and drop channels from a transmission system. With sucha technique, in order to receive or transmit data on the optical networkusing WDM, a node of the network can include at least one optical sensorthat receives the optical signal at one or more wavelengths. The opticalsensor can include an optical-electrical converter that can convert theoptical signal to electrical signals corresponding to the receivedoptical signals. Adding and/or dropping of the signals can then beperformed electronically by processing the electrical signals in theelectrical domain. The resulting electrical signal can then be modulatedonto the network using an electro-optical converter. SuchOptical-Electrical-Optical (OEO) conversion can be very complex, costlyand time consuming.

Additionally, optical wavelength add/drop multiplexers (OADM) can beused in WDM transmission systems. Currently, it has been well recognizedthat OADMs are needed to avoid the complex and costly OEO conversions.However, currently available OADMs are generally fixed. In other words,a given incoming channel (wavelength) is only associated with a fixedadd/drop port. Such a device lacks “client-configurability” andtherefore severely limits the selection of which channels to add/dropfor a client.

Therefore, there exists a need for a device to add and drop channelsfrom a transmission medium that can be readily configured according tothe needs of a client.

SUMMARY OF THE INVENTION

The invention provides an optical switch matrix device and methods thatselectively add and drop channels from an optical communication medium.The optical switch matrix can receive an input signal from an opticalmedium, such as an optical fiber cable. The input signal can includenumerous input channels, for example a plurality of channels each havinga different wavelength. The optical switch matrix can also receive anadd signal which can include numerous add channels for differentclients; each add channel can replace an input channel of the inputsignal that is dropped.

Depending on the configuration of the optical switch matrix, anychannels of the input optical signal can be dropped from thecommunication medium to any of the clients. The dropped channels can bereceived and processed by a receiver. Further, any channels from the addsignal can be added to the communication medium. The added channelsalong with the remaining channels of the input signal can then beoutputted and transmitted on the communication medium. Different fromfixed optical wavelength add/drop multiplexers (OADMs) described in therelated art, the invented optical switch matrix can be configured toallow each client to access any of the input channels, thereforeoffering client-configurability to the network.

The optical switch matrix can be a device that operates on the opticalchannels in the optical domain. For example, the optical switch matrixcan be a device, such as a micro electrical mechanical system (MEMs),having an array of micromirrors that are rotatably mounted on asubstrate. The micromirrors may be selectively positioned to interactwith passing light, so as to redirect light beams between ports of theoptical switch matrix. Accordingly, the optical switch matrix canadd/drop channels to/from an optical communication medium.

Alternatively, or in conjunction with the MEMs, the optical switchmatrix can be a device such as a matrix of switches utilizing bubbletechnology. As an optical channel passes through the optical switchmatrix, bubble switches can be selectively activated causing the channelto be redirected between ports of the optical switch matrix.Accordingly, the optical switch matrix can add/drop channels to/from anoptical communication medium.

These and other features and advantages of this invention are describedin or are apparent from the following detailed description of the systemand method according to exemplary embodiments of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The benefits of the present invention will be readily appreciated andunderstood from consideration of the following detailed description ofexemplary embodiments of this invention, when taken together with theaccompanying drawings, in which:

FIG. 1 is an exemplary block diagram of a wavelength add-drop device inaccordance with the present invention;

FIG. 2 is an exemplary block diagram of a wavelength add-drop deviceusing MEMS technology in a unidirectional network in accordance with thepresent invention;

FIGS. 3 and 4 are exemplary block diagrams of the wavelength add-dropdevice of FIG. 2 in two different functioning configurations;

FIG. 5 is an exemplary functional block diagram of a wavelength add-dropdevice according to an embodiment of the present invention;

FIGS. 6 and 7 are exemplary functional block diagrams of a wavelengthadd-drop device using MEMS technology in a bi-directional network inaccordance with the present invention; and

FIGS. 8 and 9 are exemplary functional block diagrams of a wavelengthadd-drop device using MEMS technology in a bi-directional network inaccordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an optical switch matrix system 100 for selectively addingand dropping channels from a transmission medium 120. The system 100includes an optical switch matrix 102 having four ports: input port 104,output port 106, add port 108 and drop port 110. The input port 104 isoptically coupled to a demultiplexer 112 for receiving input channels104 a-104 f from the transmission medium 120. The output port 106 isoptically coupled to a multiplexer 114 for transmitting optical channels106 a-106 f onto the transmission medium 120. The add port 108 isoptically coupled with the optical switch matrix 102 for inputting addedchannels 108 a-108 e that are connected to different clients. The dropport 110 is optically coupled with the optical switch matrix 102 fortransmitting drop channels 110 a-110 e, possibly for further processing.

Both the multiplexer 114 and the demultiplexer 112 are optically coupledwith the transmission medium 120. The transmission medium 120 caninclude any structure that allows for the transmission of opticalcommunication signals, such as an optical fiber. The opticalcommunication signals can further include a plurality of channels thatare simultaneously transmitted along the communication medium 120. Forexample, numerous channels having discrete wavelengths can be combinedonto a single optical transmission medium usingwavelength-division-multiplexing (WDM).

The demultiplexer 112 is a device that is capable of optically dividinginput signals received on the transmission medium 120 into a pluralityof channels 104 a-104 f. Once the input channel is divided, the inputchannels 104 a-104 f are transmitted to the optical switch matrix 102.As described above, the channels can travel along the transmissionmedium 120 on different wavelengths. Additionally, the channels of theinput signal can be combined on the transmission medium 120 according toany well known communication technique, such as TDMA, CDMA and the like.Any technique that allows multiple channels to be transmitted across thetransmission medium 120 and separated by the demultiplexer 112 can beused without departing from the spirit and scope of the presentinvention.

The multiplexer 114 is a device that is capable of optically combiningthe output channels 106 a-106 f received from the optical switch matrix102 into an output signal that is then transmitted on the transmissionmedium 120. As described above, the numerous output channels 106 a-106 fcan travel along the transmission medium as an output signal inaccordance with any known or later developed transmission techniquewithout departing from the spirit and scope of the present invention.

The add port 108 is a device that is capable of receiving channels 108a-108 e from different clients, and then transmitting the added channels108 a-108 e to the optical switch matrix 102. Data sources for the addedchannels 108 a-108 e can be generated by a plurality of light sources,such as tunable laser diodes, included in the add port 108. Each of thelight sources can be adjusted to emit a channel having a specificwavelength. The light sources of the add port 108 can further operate inaccordance with instructions received from a controller (not shown) inorder to selectively output an added channel of a specific wavelength.For example, added channels 108 a-108 e can each be transmitted ondifferent wavelengths λ_(a)-λ_(f), corresponding to the wavelengths ofthe input channels.

The drop port 110 is a device that is capable of receiving drop channels110 a-110 e from the optical switch matrix 102. Each of the channels canbe of various wavelengths. The drop port 110 can then output any of thedrop channels to a processor (not shown) for further processing.

The optical switch matrix 102 is a device that is capable of redirectingoptical signals passing through the optical switch matrix 102. In thismanner, a portion of the input channels 104 a-104 f can pass through theoptical switch matrix 102 to the output channels 106 a-106 f without anysubstantial interference. In other words, these channels are permittedto pass nearly unabated through the optical switch matrix 102 andcontinue to travel on the transmission medium 120.

Alternatively, a portion of the input channels can be selectivelyredirected to a drop channel 110 a-110 e of the drop port 110 as theinputted channels 104 a-104 f pass through the optical switch matrix102. In a similar manner, added channels 108 a-108 e can be selectivelyredirected to output channels 106 a-106 f of the output port 106 forwhich the corresponding input channel has been dropped as the addedchannels pass through the optical switch matrix 102. According to thistechnique, input channels can be removed/dropped and new channels can beadded to the transmission medium 120.

The optical switch matrix 102 can include an array of switches that canbe in either an active or inactive state. In an active state, the switchis able to redirect a light beam or channel passing in close proximityto the switch. In an inactive state, the switch allows a light beam orchannel to pass without incident.

As an example of operation, assume that the optical switch matrix 102includes at least N×M matrix of switches that are initially in theinactive position. Further assume that the transmission medium 120 istransmitting an input signal having 6 channels (A-F). In the initialstate, the input signal can be received by the demultiplexer 112. Thedemultiplexer 112 operates on the input signal to optically separate theinput signal into input channels 104 a-104 f. The input channels 104a-104 f are then transmitted to the optical switch network 102.

In the initial state of the switch matrix 102, where all of the opticalswitches are in the inactive state, the input channels 104 a-104 f arepermitted to pass through the optical switch matrix to the outputchannel 106 a-106 f without being acted upon. Accordingly, the outputchannels 106 a-106 f, corresponding to the input channels 104 a-104 fare transmitted to the multiplexer 114. The multiplexer 114 thenoptically operates on the output channels 106 a-106 f in order tocombine the output channels 106 a-106 f into an output signal, and thentransmit the output signal back onto the transmission medium 120.

During the course of operation, assume that it has now become desirableto replace input channel 104 c with an added channel 108 b. Accordingly,as the input channels 104 a-104 f are transmitted through the opticalswitch matrix 102, one or more optical switches in the path of inputchannel 104 c could be switched to an active state whereby the opticalswitch can redirect the light beam corresponding to input channel 104 cto the specified drop port 110, such as dropped signal 110 a.Furthermore, the add port 108 can begin transmitting an added signal 108b into the optical switch matrix 102 and an optical switch in the pathof added channel 108 b could be switched to an active state, and therebyredirect the added channel 108 b to output channel 106 c of the outputport 106.

Accordingly, the multiplexer would then receive the input channel 104 aon output channel 106 a, the input channel 104 b on the output channel106 b, the added channel 108 b on the output channel 106 c, the inputchannel 104 d on the output channel 106 d, the input channel 104 e onthe output channel 106 e and the input channel 104 f on the outputchannel 106 f. The output channels 106 a-106 f would then be combined bythe multiplexer 114 and transmitted as an output signal across thetransmission medium 120. In this manner, a channel of the input signal,104 c, has been replaced (dropped) during the addition of the addedchannel 108 b.

As is to be understood, the switches of the optical switch matrix 220can be changed at any time during operation to add or drop channels toor from the transmission medium 120. In this manner, a user can easilyconfigure the optical switch matrix 102 to add or remove all or aportion of information from an optical network.

As shown in FIG. 2, the optical switch matrix 102 can be a singlemicroelectrical mechanical system (MEMs). This single MEMs design of theoptical switch matrix can be particularly useful for dropping and addingchannels from an unidirectional ring network. The MEMs includes an arrayof micromirrors 280 that are rotatably mounted to a substrate 282. Themicromirrors 280 are rotatable between a non-activated and activatedposition. In the non-activated position, the micromirrors 280 aresubstantially parallel and flush with the substrate 282. In the activeposition, the micromirrors 280 are rotated or flipped to be in asubstantially perpendicular position relative to the substrate 280.Furthermore, in the active position the micromirrors 280 ate positionedwithin the light path of channels passing through the optical switchmatrix.

This type of optical switch matrix is discussed in detail in Journal ofMicroelectro-mechanical Systems, Vol. 5, No. 4, December 1996, entitled“Electrostatic Micro Torsion Mirrors for an Optical Switch Matrix” byHiroshi Toshiyoshi and Hiroyuki Fujita, incorporated herein by referencein its entirety. The optical switch matrix is also discussed inco-pending and commonly assigned patent application Ser. No. 09/002,240filed on Dec. 31, 1997, also incorporated herein by reference in itsentirety.

As shown in FIG. 2, the wavelength add-drop device 200 includes anoptical N×M matrix switch 220 coupled to an input demultiplexer 210, anoutput multiplexer 230, an add port 240 and a drop port 250.

The optical N×M matrix switch 220 can be a four-port matrix switch. Inthis embodiment, the optical N×M matrix switch 220 is a N×M free spaceMEMS crossconnect that comprises N×M micromirrors 280. In the exemplaryembodiment shown in FIGS. 2-4, N=6 and M=5. As described above, each ofthe 30 micromirrors 280 shown in FIGS. 2-4 may take one of an active orinactive position.

The positions of the micromirrors 280 can be controlled by a matrixcontroller (not shown in FIGS. 2-4). By energizing the switch that is onthe i^(th) row and the j^(th) column of the matrix switch, e.g., byflipping up the micromirror switch on line i and column j, andconcurrently tuning the light source on 241 j to the wavelength used onthe i^(th) line, one can thus add a wavelength from light source 241 jand/or drop a wavelength at sensor 251 j.

Although the optical switch matrix 102 of FIG. 1 has been described inthe exemplary embodiments of FIGS. 2-4 as being a MEMs type switch 220,it is to be understood that various other switches can be used withoutdeparting from the spirit and scope of the present invention. Forexample, the optical switch matrix 102 can be any type of optical switchwith or without a micromechanical element, such as optical switchesbased on total internal reflection of a fluid-containing planar lightwave circuit (PLC), otherwise known as bubble technology. Suchtechnology is more fully described in the article entitled “CompactOptical Cross-Connect Switch based on Total Internal Reflection in aFluid-Containing Planar Light Wave Circuit” by J. E. Fouquet, in 2000OFC Technical Digest, pp. 204 to pp. 206, which is incorporated hereinby reference in its entirety.

Referring again to FIG. 2, as an example of operation, assume that in aninitial state of operation, all of the micromirrors 280 are in theinactive position. Next, assume that a determination has been made thata signal conveyed by the input line 260 is to be dropped. If so, it isdetermined on which line of the matrix the light ray that has thewavelength that carries the signal to be dropped is transmitted. Next,the sensor 251A to 251M on which the signal is to be received isdetermined. The micromirror 280 corresponding to that line and thecolumn of that selected sensor in the matrix is next positioned in theactive position. Next, a determination is made whether another signalconveyed by the input line 260 has to be dropped. The above operationsare repeated until no other signal conveyed by the input line 260 has tobe dropped.

As mentioned above, since the N×M matrix switch in FIG. 2 is implementedin a unidirectional network, the add channels are always associated withthe drop channels. That is, the add channel and drop channel on the samecolumn are associated with the same client. By using the backsidereflection of the activated micromirrors, and concurrently tuning thelasers of the add channels to selected wavelengths, signals can be addedinto traffic from the selected add channels.

FIGS. 3 and 4 are exemplary functional block diagrams of the wavelengthadd-drop device of FIG. 2 in two different functioning configurationscorresponding to a same set of dropped light rays in a unidirectionalnetwork. In FIGS. 3 and 4, the dropped light rays are the lights raysemitted by input ports 211B and 211D, on the second and fourth lines ofswitch matrix 220. However, in the configuration outlined in FIG. 3, thelight ray transmitted on the second line is dropped to the sensor 251Dand the light ray transmitted on the fourth line is dropped to thesensor 251B. In the configuration outlined in FIG. 4, the light raytransmitted on the second line is dropped to the sensor 251C and thelight ray transmitted on the fourth line is dropped to the sensor 251E.

Consequently, in the configuration outlined in FIG. 3, the added signalsare added by inputting light rays from the light sources 241B and 241D.In the configuration outlined in FIG. 4, the added signals are added byinputting light rays from the light source 241C and 241E.

FIGS. 3 and 4 show that the wavelength add-drop device according to anexemplary embodiment of the invention can be configured to select whichsensor 251A-251E receives the dropped signal and to select the lightsource 241A-241E that inputs the added signal.

The present invention describes a device that offers fullclient-configurability, permitting any subset of the incomingwavelengths (1, 2, . . . N) to be added or dropped at any subset of thelight sources 241A to 241E or sensors 251A to 251E.

An exemplary wavelength add-drop device 400 is outlined in FIG. 5, andincludes a signal manager 410, the optical N×M matrix switch 102, theinput demultiplexer 112, the output multiplexer 114, the add port 108,the drop port 110, the input line 160 and the output line 170.

The signal manager 410 comprises a dropped signal processor 420, amatrix controller 430 and an added signal processor 440. The droppedsignal processor receives the signals output by the dropped channels 110a-110 e of the drop port 110 and processes those signals. The matrixcontroller 430 determines which micromirrors in the optical matrixswitch 102 are to be turned to their active position and commands thepositions of the micromirrors. The added signal processor 440 providesthe signals to be added through the add port 108 and the light sources108A-108E.

The signal manager 410, the dropped signal processor 420, the matrixcontroller 430 and the added signal processor 440 may be, in theexemplary embodiment of the invention shown in FIG. 5, a microprocessorthat uses software to implement exemplary embodiments of the methods anddevices according to this invention.

FIG. 6 shows a wavelength add-drop device 700 wherein the optical switchmatrix 102 includes two optical N×M matrix switches 720A and 720B. Thisconfiguration of the optical switch matrix 102 having two MEMs can beparticularly useful for adding and dropping channels from abi-directional ring network and/or a linear network. The optical N×Mmatrix switch 720A is coupled to an input demultiplexer 710, a drop port750 and the optical N×M matrix switch 720B. The optical N×M matrixswitch 720B is also coupled to an output multiplexer 730 and an add port740.

As described above, the embodiment of the present invention described inFIG. 6 may be used in combination with a linear network or abi-directional ring. Dropping and adding optical signals may be carriedout independently by the optical N×M switch matrices 720A and 720B whichprovide full client-configurability since the add port and the drop portassociated with the same input channel or wavelength are independent ofeach other.

FIG. 7 shows an example of operation of the embodiment described in FIG.6. In this example, assume that it is desired to replace a channelcorresponding to input channel 711A with a new channel corresponding toadded channel 741C. As described above, the input channel is received bythe demultiplexer 710. The demultiplexer divides the channels intorespective input channels 711A-71F that are then input into the opticalswitch matrix 102.

As can be seen in FIG. 7, the first MEMs 720A of the optical switchmatrix 102 receives the input channels. Further, micromirror 722 has nowbeen switched to an active state. Accordingly, the input channel 711A isredirected to a output channel 751B of the drop port 750. Additionally,as can be seen, the remaining input channels 711B-711F are permitted topass across the first MEMs 720A without interference.

Simultaneous to the dropping of input channel 711A, an input signal 741Cis added to the output channel 731A by the add port 740. As can be seen,a micromirror 724 of the second MEMs 720B is switched into an activeposition. Accordingly, the input signal 741C is redirected to the outputport 731A of the output multiplexer 730. Additionally, the output ports731B-731F receive the input channels 711B-711F, respectively.

The multiplexer 730 then combines the new combination of output channels731A-731F into an output signal. The output signal is then transmittedacross the transmission medium 770. Accordingly, the channelcorresponding to input channel 711A has been removed from thetransmission medium and the channel corresponding to added channel 741Chas been added in the removed channel's place.

It should be noticed that in the embodiment of the present inventionoutlined in FIG. 6, only one side of the switching mirrors is used.Moreover, the structure shown in FIG. 6 is strictly non-blocking. Inother words, a new connection or a connection change can be made withoutrerouting the existing non-changing connections.

FIG. 8 shows a wavelength add-drop device wherein the optical switchmatrix 102 includes one optical N×M matrix switch 820A and one opticalM×M switch 820B. The optical N×M matrix switch 820A is coupled to aninput demultiplexer 810, a drop port 850, the optical M×M matrix switch820B and an output multiplexer 830. The optical M×M matrix switch 820Bis coupled to an add port 840.

The embodiment of the present invention described in FIG. 8 may also beused in combination with a linear network or a bi-directional ringnetwork. Dropping and adding optical signals may be carried outindependently by the optical matrix switches 820A and 820B which providefull client-configurability since the add port and the drop portassociated with the same input channel or wavelength are independent ofeach other due to the optical M×M matrix 820B.

FIG. 9 shows an example of operation of the embodiment described in FIG.8. In this example, assume that it is desired to replace a channelcorresponding to input channel 811A with a new channel, corresponding tothe added channel 841D. As described above, the input signal is receivedby the demultiplexer 810 and separated into input channels 811A-811F. Asthe input channel 811A is transmitted across the MEMs 820A, the inputchannel 811A's path is obstructed by micromirror 822 which is in anactivated position. A front side 822 a of micromirror 822 causes theinput channel 811A to be redirected to drop channel 851B.

Additionally, as the input channel 811A is being dropped, the addedchannel 841D is being added. The added channel originates from the addport 840 and is transmitted across the MEMs 820B until it is redirectedby activated micromirror 824. The micromirror 824 redirects the addedchannel 841D so that it intersects with an opposite side 822 b of themicromirror 822. The opposite side 822 b of the reflecting mirror 822redirects the channel 841D to the output channel 831A of the multiplexer830.

As can be seen from the example described above, the input channels811B-811F will be transmitted across the MEMs 820A and be received bythe corresponding output channels 831B-831F. The input channel 811A willbe dropped to the drop port 851B, while the added channel 841D will betransmitted to the output port 831A. Accordingly, the output multiplexer830 will combine the individual channels into an output signal andtransmit it across the transmission medium 870.

It should be appreciated that, in the embodiment of the presentinvention outlined in FIGS. 8 and 9, two sides of the switching mirrorsof the optical N×M switch matrix 820A are used but only one side of theswitching mirrors of the optical M×M switch matrix 820B are used.Moreover, the structure shown in FIGS. 8 and 9 is strictly non-blocking.A new connection or a connection change can be made without reroutingthe existing non-changing connections.

While this invention has been described in conjunction with theexemplary embodiments outlined above, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, the exemplary embodiments of theinvention, as set forth above, are intended to be illustrative, notlimiting. Various changes may be made without departing from the spiritand scope of the invention.

1. An optical switching device, comprising: an optical switch matrixhaving one or more optical switches that are capable of redirectingoptical channels passing therethrough; an input port coupled to theoptical switch matrix that receives at least one input channel andtransmits the at least one channel to the optical switch matrix; anoutput port coupled to the optical switch matrix that receives at leastone output channel from the optical switch matrix; an add port coupledto the optical switch matrix that inputs add channels to the opticalswitch matrix, each said add channel being tuned to a selectedwavelength by said add port; and a drop port coupled to the opticalswitch matrix that receives dropped channels from the optical switchmatrix; wherein the switches of the optical switch matrix can beselectively configured so that the at least one input channel isdirected to the drop port and at least one add channel is directed tothe output port; wherein the optical switch matrix includes a firstarray of switches and a second array of switches; and wherein the firstarray of switches and the second array of switches are respective N×Marrays of switches, and the input port and the drop port are coupled tothe first array of switches, and the add port and the output port arecoupled to the second array of switches.
 2. The optical switching deviceof claim 1, wherein the optical switch matrix is a microelectricalmechanical system having an array of micromirrors arranged on asubstrate.
 3. The optical switching device of claim 2, wherein the firstarray of switches redirects optical channels from the input port to thedrop port, and the second array of switches redirects optical channelsfrom the add port to the output port.
 4. An optical switching device,comprising: an optical switch matrix having one or more optical switchesthat are capable of redirecting optical channels passing therethrough;an input port coupled to the optical switch matrix that receives atleast one input channel and transmits the at least one channel to theoptical switch matrix; an output port coupled to the optical switchmatrix that receives at least one output channel from the optical switchmatrix; an add port coupled to the optical switch matrix that inputs addchannels to the optical switch matrix, each said add channel being tunedto a selected wavelength by said add port; and a drop port coupled tothe optical switch matrix that receives dropped channels from theoptical switch matrix; wherein the switches of the optical switch matrixcan be selectively configured so that the at least one input channel isdirected to the drop port and at least one add channel is directed tothe output port; wherein the optical switch matrix includes a firstarray of switches and a second array of switches; and wherein the firstarray of switches is an M×M array of switches, the second array ofswitches is an N×M array of switches, the add port is coupled to thefirst array of switches and the input port, output port, and drop portare coupled to the second array of switches.
 5. The optical switchingdevice of claim 4, wherein the optical switch matrix is amicroelectrical mechanical system having an array of micromirrorsarranged on a substrate.
 6. The optical switching device of claim 5,wherein an input channel is re-directed to a drop port by a frontsurface of a first micromirror of the N×M array of switches, and an addchannel is redirected to an output port by a front surface of a secondmicromirror of the first array of switches and a back surface of thefirst micromirror of the second array of switches.
 7. Apparatuscomprising: a first row-and-column optical switch array and a secondrow-and-column optical switch array, the columns of said first andsecond optical switch arrays being aligned with one another, an inputport and an output port, said input port being adapted to launch aplurality of input channels along respective rows of said first opticalswitch array to said output port, an add port adapted to launch addchannels along respective rows of said second optical switch array, saidsecond optical switch array being operable to divert any of said addchannels along any of the columns of said second optical switch array tothe aligned column of said first optical switch array, said firstoptical switch array having a micromirror at each row/columnintersection, each said micromirror having a reflective first surfaceand a reflective second surface and being operable to reflect an inputchannel from said first reflective surface and thereby divert that inputchannel from its respective row, and to reflect an add channel from saidsecond surface and thereby divert that add channel from its respectivecolumn onto the row of the diverted input channel.
 8. The invention ofclaim 7 wherein said first and second surfaces are front and backsurfaces, respectively.
 9. Apparatus comprising: a first optical switcharray comprising a plurality of micromirrors each having a reflectivefirst surface and a reflective second surface, a second optical switcharray, an input port adapted to launch a plurality of input channels atrespective ones of a plurality of wavelengths into said first opticalswitch array, an output port, said input port, said output and saidfirst optical switch array being such that each of said input channelscan pass through said first optical switch array to said output port, atleast an individual one of said micromirrors being operable to cause arespective input channel to be reflected off the first surface of thatmicromirror and be thereby diverted from said output port, and an addport adapted to launch add channels into said second optical switcharray, said add port and said first and second optical switch arraysbeing such that, and said second optical switch array being operablesuch that, any of said add channels can be directed to the secondsurface of any operated one of said micromirrors and thereby be directedto said output port at any of said wavelengths.
 10. The invention ofclaim 9 wherein said first and second surfaces are front and backsurfaces, respectively.